The present invention relates to a beam shaping optical system that affects a cross-sectional shape of a light beam, and a multi-beam optical system employing the beam shaping optical system.
A laser diode emits a beam having an elliptical-shaped far-field pattern. To convert the elliptical-shaped light beam to a beam having a circular cross section, conventionally, a beam shaping optical system has been used. The conventional beam shaping optical system is provided with a single or a pair of wedge prisms, which are aligned such that a principal section thereof is in parallel with a direction of a semiminor axis of the elliptical-shape of the beam. It should be noted that the principal section of a wedge prism is defined as a plane perpendicular to both of the two refractive surfaces thereof through which a light beam passes. By the above arrangement, the semiminor axis of the elliptical beam is magnified by the wedge prism to have substantially the same length of the semimajor axis thereof.
This type of beam shaping optical system is generally included in an optical system of a laser beam printer or the like in order to control light output by a light source such as a laser diode effectively to form a symmetrical beam spot on an object surface.
Further, in order to increase the scanning efficiency of such devices (laser beam printers and the like), a plurality of light beams may be simultaneously used to read or write information. In such a case, a multi-beam laser diode is generally used as a light source. A multi-beam laser diode includes a plurality of light emitting points arranged in a single element.
However, if the conventional beam shaping optical system which is designed for converting the cross-sectional shape of a single light beam is used for the multi-beam optical system, a problem arises in that, due to a variation in incident angles onto the wedge prism among the beams emitted from the multiple emitting points of the multi-beam laser diode and collimated by a collimating lens, intervals between the spots formed on the objective surface (i.e., a surface on which the spots are to be formed) cannot be maintained constant. Specifically, the light emitting points of the multi-beam laser diode are aligned in a direction in parallel with the semiminor axes of the elliptical shape of the emitted beams. Therefore, in order to magnify the semiminor axis of the elliptical cross section of each beam, it is preferable that the light emitting points of the multi-beam laser diode are aligned in a direction parallel to the principal section of the wedge prism. However, since an angular magnification by the beam shaping optical system in the principal section varies depending on the incident angle of an incident beam, emitting angles of the plurality of beams are different from incident angles thereof. Therefore, when the conventional beam shaping optical system is used for a multi-beam optical system employing a multi-beam laser diode, beam spots formed on the objective surface may be positioned at irregular intervals even if the emitting points of the light beams are positioned at predetermined regular intervals.